1. Field of the Invention
The present invention relates to front wheel drive transaxle units for a motor vehicle, and more particularly to a front wheel drive transaxle unit having a differential assembly and a torque-coupling device providing the differential assembly with a limited slip capability.
2. Description of the Prior Art
Conventionally, differentials well known in the prior art, are arranged in a power transmission system of a motor vehicle to allow a pair of output axle shafts operatively coupled to an input shaft to rotate at differential speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, these types of differentials known in the art as an open differentials, i.e. a differential without clutches or springs, are unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a “spin out” of that wheel. Since the maximum amount of torque, which can be developed on the wheel with traction, is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. Thus, the necessity for a differential, which limits the differential rotation between the output shafts to provide traction on slippery surfaces, is well known.
Limited-slip differentials are a relatively common option for rear-wheel-drive (RWD) vehicles, but are non-existent for front-wheel-drive (FWD) vehicles. The FWD vehicles typically utilize transaxles with open differentials.
Given the increased emphasis on high performance FWD vehicles in recent years, there is an increasing need for a controllable limited-slip differential for FWD transaxle units. Packaging the limited-slip differential in the FWD transaxle unit is extremely difficult due to space limitations, and the FWD transaxle housings cannot be economically redesigned to allow for more space.
Thus, what is needed is a FWD transaxle unit having a differential assembly capable to provide a limited-slip function only when required, i.e. limited-slip when one wheel has lost traction, and perform as an open differential when sufficient torque is developed.